<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        p{
            font-size: 12px;
            color: red;
            margin: 0;
        }
        img{
            position: absolute;
            right: 30px;
            top: 15px;
        }
       #i1{
           border: none;/*去掉边框*/
           width: 260px;
           padding: 10px 20px;
        }
       div{
           width: 300px;
           background-color: rgba(0,0,0,0.3);
           padding: 10px;
           position: relative;/*做参照物*/
       }
    </style>
</head>
<body>
<div>
    <input type="text" placeholder="请输入用户名" id="i1">
    <img src="http://doc.canglaoshi.org/tstore_v1/images/login/yhm.png" alt="">
    <p>用户名不能为空!</p>
</div>
</body>
</html>